Components, including electrical devices and other items to be placed on printed circuit boards (PCB) and the like, are often supplied within a pocket carrier tape that has been wound onto a reel for mounting within a component feeder. The carrier tape consists of two parts; (1) a flexible base of paper, metal or plastic having depressions or pockets in series at regular intervals along its length and (2) a thin continuous film forming an over-cover of the component pocket. The pocket tape also includes a plurality of through-holes spaced along the lengthwise edge at a predefined pitch, generally on the order of 2 mm, wherein a drive roller is able to engage the holes and advance the tape a controlled distance so as to accurately position the pocket at the component retrieval station or pick point within the feeder mechanism.
Placed within each pocket is one component to be retrieved and positioned onto the PCB by means of a robotic arm within the assembly machine. The components are mechanically and hermetically secured within their respective pockets using a thin transparent material, commonly referred to as “cover tape” that extends the entire length of the pocket tape, and is just slightly wider than the pocket that it covers. The cover tape is typically fastened or affixed to the pocket tape with a pressure or temperature activated adhesive along each of its longitudinal edges, thus forming the combination of the cover tape and the pocket tape to provide a serial string of sealed component vessels.
In component tape feeders the tape feeder advances the pockets to a pre-defined position commonly referred to as a “pick point” or “pick-up location”. As the component is advanced from of a supply reel, the cover tape is typically pulled and peeled back from the pocket tape, just prior to the pick point, so as to expose the component contained therein and thus provide a direct access to the component by a pick up-nozzle located within the pick and place printed circuit board assembly machine.
Therefore it can be appreciated that component tape feeders are an essential element for the orderly, rapid and sequential delivery of individual surface-mountable components to an assembly machine having an automated or robotic head for retrieving the component from the pocket and placing the component onto the surface of a printed circuit board being assembled.
A long-standing problem with this technology has been the disposal of the two segregated elements of the used component tape; the cover tape and the pocket tape portions. The base segment of the component carrier tape, being the rather bulky pocket tape, is generally expelled directly out the bottom of the component feeder into a large waste bin where it is often chopped into short lengths for subsequent disposal. The spent cover tape, on the other hand, is peeled in the upper region of the component feeder and is physically segregated from the pocket tape waste bin by the incoming component carrier tape and the feeder mechanism. Accordingly, a discrete cover tape waste collection method and system is required to be located within the upper area of the feeder.
Two principal methods are currently employed for collecting the waste cover tape. The first is to wind the cover tape on a take-up reel located above and just behind the pick point. A known problem with this method is that in order for the take-up reel to be re-used a full take-up reel must be manually unwound so as dispose the tape and reclaim the take-up reel. In addition, the leading edge of the cover tape must be threaded and cinched within the hub of the take-up reel each time the take-up reel is emptied.
A second method of handling the cover tape, once it is peeled back from the carrier tape, involves the use of a pair of pinch rollers to push the cover tape into a waste collection reservoir, which is generally emptied each time a replacement reel of components is loaded into the feeder. While such a system resolves the issues of attaching and the un-winding of the cover tape with the take-up reel method, it still requires the operator to routinely remove and dispose of the cover tape each time the feeder is re-loaded, and in some cases between re-loading wherein the volume of the waste reservoir may be insufficient to retain the cover tape removed from a reel of components, thus resulting in an interruption of the assembly process to allow for operator intervention to empty the cover tape out of the reservoir. In the alternative, it is not uncommon for the reservoir to retain more than one reel of cover tape. Given the variances of parts/reel and the variety of cover tape composition there is little to no synchronization between a reel of components and cover tape reservoir capacity.
An apparent solution to decreasing the frequency of empting the used cover tape would be to increase the capacity of the reservoir by simply increasing the volume. However, in a majority of the feeders there are design limitations that restrict the footprint of the reservoir. Accordingly, attention is directed to the other side of the volume equation, that is to improve the packing density of the cover tape in the reservoir while the volume remains a constant.
Inherently cover tape is difficult to manage for a few reasons; (1) it has very limited beam strength, (2) it readily acquires a charge and, therefore, suffers bi-polar attraction or “static cling” and (3) it contains residual pressure sensitive adhesive (PSA) that can readily migrate to the inner surface of the reservoir and contaminate the cover tape transport. While there are various solutions for static dissipation issues, the problem of pushing a large quantity of insubstantial and sticky thin film into a reservoir remains a pressing issue.
The present invention addresses the aforementioned cover tape collection problems described above by (1) controlling the beam strength of the thin film cover tape, and (2) mitigating PSA migration within the feeding apparatus. Additionally, it was discovered, that variations to the geometric profile and an innovative surface finish of the reservoir proved to be enabling in enhancing the packing density of the reservoir as measured in linear inches of cover tape per cubic inch volume of the collection reservoir, or D=in/in3 where D is a unit of packing density expressed in inches of cover tape.
One aspect of the invention deals with the basic problem of controlling the bending moment of a cover tape, having a planar cross sectional thickness of approximately 0.0025 inches, when subjected to the compressive force required to push the tacky thin film web into a collection reservoir. The resultant of handling flimsy material in this manner is a localized mass of balled-up tape positioned in direct proximity of the entrance to the reservoir. Eventually a jam will occur where the tape back feeds and wraps around the rollers of the cover tape drive system, thus jamming the mechanism and preventing additional cover tape from being pushed into the reservoir. In a 3M Technical Bulletin dated November 2001 entitled “Prevention of Pressure Sensitive Adhesive Cover Tape Jamming in Feeders with Nip Gear and/or Collection Bins Systems” there is disclosed an arrangement of two or more rollers that fold the edges of the cover tape, along the longitudinal sides of the cover tape, back onto themselves and thereby encapsulate the PSA therebetween. However it is notable that this process also produces a 2-ply cover tape resulting in a substantial increase in column strength and thickness. This approach works well in isolating the PSA, and provides rigidity to assists in the pushing of the cover tape into the reservoir. However, upon further analysis, it was discovered that this added stiffness of the cover tape may actually limit the ability for the tape to freely flow within the reservoir due to the lack of compliance of the cover tape within the confined perimeter of the reservoir. Preferably, upon the tape exiting the drive mechanism and just entering the reservoir, it would be advantageous to have the thin film respond as if it were again in a “liquid state”, that is to say, the cover tape should preferably react within the cover tape drive gears as a rigid member and then experience a change in state upon entering the reservoir, whereas the tape should desirably respond as if it was being “poured” into the reservoir cavity, thus reducing voids or air pockets within the reservoir. Accordingly, the present invention discloses a unique process whereby the cover tape drive gears corrugate the cover tape and thus provide for an unanticipated improvement in packing density.
In accordance with another aspect of the present invention, consecutive component carrier tapes are often connected together by overlapping the ends and adhering or splicing one to another with splicing tape applied on one side only. Component tape splicing is an evolving process in the industry that provides for a continuous supply of components, without having to stop and reload a new component tape into the feeder. However, when folding and bending the cover tape within a folding pulley the splice and the cover tapes have a tendency to peel apart during the edge folding operation. Accordingly there has been provided in one embodiment of the present invention a splice deflector operatively associated with the folding pulley mentioned above. While the splice material has a propensity to delaminate from the cover tape when a transverse bending moment and a longitudinal rotational force is applied to the cover tape, the splice deflector provides a holding force along the top side of the cover tape to guide the protruding end of the cover tape as the splice is drawn through the folding pulley.
In accordance with one aspect of the present invention, there is provided a distinctive geometric contour of the perimeter of the cover tape reservoir that promotes a laminar flow during the consolidation of the cover tape into a uniform mass. This aspect is based on the discovery that curvilinear profiles strategically placed along the perimeter of the reservoir provides for a more homogeneous and uniform collection of the cover tape, thereby avoiding, or at best reducing, the areas of low packing density within the reservoir.
In accordance with another aspect of the present invention, there is provided a cover tape reservoir having formed on at least one interior surface a texture or treatment that minimizes the adhesion and/or friction between the cover tape and the stationary sides of the reservoir. This surface profile has a substantially irregular and random finish that minimally contacts the cover tape.
In accordance with yet another aspect of the present invention there is provided a single folding pulley having undercut inclined flanges with a width less than the width of the tape so as the edges of the tape are encourage to fold inwardly onto themselves.
In accordance with yet another aspect of the present invention, there is provided a cover tape drive system comprising a driven gear mechanically engaged to a spring tensioned normal force idler gear. The tension, which provides the force to peel the cover tape from the pocket tape, is imparted to the cover tape by passing through the nip point of the engaging gears resulting in a pulling, as well as pushing, force onto the cover tape. While the distortion of the cover tape engaged between the meshing gears limits slippage, an additional benefit provides for the “corrugating” of the tape into minute segments, whereby individual segments have notable column strength afforded by the previously mentioned folding process. However when taken together as a continuous strip the entire corrugated tape has virtually no column strength, as is the case with most linked members, for example, a chain having rigid links with fulcrums therebetween. This corrugating or pleating of the cover tape has been found to be advantageous based on the discovery that once the drive gears have expelled the cover tape past the nip of the gears, the corrugating of the tape allows for it to be compliant within the remaining space available in the reservoir. In principle the tape is forcibly inserted into the reservoir one “link” or segment at a time as the tape freely fills the reservoir, the unanticipated resultant being a significant increase in packing density
In accordance with yet another aspect of the present invention, there is provided a system for automatically advising the operator of a relatively full reservoir. This technique relies on the real-time measurement of the component tape length and then comparing this measurement to a pre-determined reservoir capacity table, where various levels of fullness are set forth (e.g., “almost full”=0.9×N inches and “full to capacity”=N inches). A carrier tape drive, via an encoder, records in software the amount of cover tape that has passed through the feeder since the reservoir was last emptied. An algorithm in software empirically determines the point where, based upon the measured length and the aforementioned table, the reservoir is “almost full” and the operator is initially signaled, or “full to capacity” where the operator is required to evacuate the cover tape from the reservoir in order to resume operation of the feeder. A sensor resets the software gauge once the reservoir has been emptied and the access door closed.
In accordance with an alternative to the software algorithm for gauging the amount of tape contained within the reservoir, there is provided an electromechanical means to monitor the packing density of the cover tape within the reservoir. The current (milliamps) drawn by the DC motor driving the cover tape drive gears is measured and correlated to a current versus. packing density table contained within the software. Fundamentally the cover tape in the reservoir provides a back force that progressively increases the amount of torque, and therefore current, required to push on the cover tape as it fills the reservoir. This invention is based on the discovery that the load on the DC motor varies appreciably as a function of the amount of cover tape contained within the reservoir. An empirically derived table, based on the direct relationship between current and voltage, provides the following algorithm for feedback in order to estimate “fullness” where; Torque (T)=Current (I)×Voltage (E). Assuming the voltage (E) supplied to the motor remains a constant, the output torque of the motor is directly proportional to the current (I), measured in milliamps, as tape is forced into the reservoir. Within the current versus reservoir table there are at least two trip points, one representing a status A, almost full and status B, a full reservoir. Status A is communicated to the operator as a warning, whereas status B results in a halt of feeder operation until the cover tape has been removed from the reservoir.
The techniques described herein are advantageous because they provide simple and reliable approaches to the disposition of used cover tape from a component carrier tape. The techniques are flexible and can be adapted to any of a number of component tape feeders.
The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.